2,289 research outputs found

    Initial synchronisation of wideband and UWB direct sequence systems: single- and multiple-antenna aided solutions

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    This survey guides the reader through the open literature on the principle of initial synchronisation in single-antenna-assisted single- and multi-carrier Code Division Multiple Access (CDMA) as well as Direct Sequence-Ultra WideBand (DS-UWB) systems, with special emphasis on the DownLink (DL). There is a paucity of up-to-date surveys and review articles on initial synchronization solutions for MIMO-aided and cooperative systems - even though there is a plethora of papers on both MIMOs and on cooperative systems, which assume perfect synchronization. Hence this paper aims to ?ll the related gap in the literature

    UWB communication systems acquisition at symbol rate sampling for IEEE standard channel models

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    For ultra-wideband (UWB) communications, acquisition is challenging. The reason is from the ultra short pulse shape and ultra dense multipath interference. Ultra short pulse indicates the acquisition region is very narrow. Sampling is another challenge for UWB design due to the need for ultra high speed analog-to digital converter.A sub-optimum and under-sampling scheme using pilot codes as transmitted reference is proposed here for acquisition. The sampling rate for the receiver is at the symbol rate. A new architecture, the reference aided matched filter is studied in this project. The reference aided matched filter method avoids using complex rake receiver to estimate channel parameters and high sampling rate for interpolation. A limited number of matched filters are used as a filter bank to search for the strongest path. Timing offset for acquisition is then estimated and passed to an advanced verification algorithm. For optimum performance of acquisition, the adaptive post detection integration is proposed to solve the problem from dense inter-symbol interference during the acquisition. A low-complex early-late gate tracking loop is one element of the adaptive post detection integration. This tracking scheme assists in improving acquisition accuracy. The proposed scheme is evaluated using Matlab Simulink simulations in term of mean acquisition time, system performance and false alarm. Simulation results show proposed algorithm is very effective in ultra dense multipath channels. This research proves reference aided acquisition with tracking loop is promising in UWB application

    Packet CDMA communication without preamble

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    Code-Division Multiple-Access (CDMA) is one of the leading digital wireless communication methods currently employed throughout the world. Third generation (3G) and future wireless CDMA systems are required to provide services to a large number of users where each user sends data burst only occasionally. The preferred approach is packet based CDMA so that many users share the same physical channel simultaneously. In CDMA, each user is assigned a pseudo-random (PN) code sequence. PN codephase synchronization between received signals and a locally generated replica by the receiver is one of the fundamental requirements for successful implementation of any CDMA technique. The customary approach is to start each CDMA packet with a synchronization preamble which consists of PN code without data modulation. Packets with preambles impose overheads for communications in CDMA systems especially for short packets such as mouse-clicks or ATM packets of a few hundred bits. Thus, it becomes desirable to perform PN codephase synchronization using the information-bearing signal without a preamble. This work uses a segmented matched filter (SMF) which is capable of acquiring PN codephase in the presence of data modulation. Hence the preamble can be eliminated, reducing the system overhead. Filter segmentation is also shown to increase the tolerance to Doppler shift and local carrier frequency offset. Computer simulations in MATLABÂź were carried out to determine various performance measures of the acquisition system. Substantial improvement in probability of correct codephase detection in the presence of multiple-access interference and data modulation is obtained by accumulating matched filter samples over several code cycles prior to making the codephase decision. Correct detection probabilities exceeding 99% are indicated from simulations with 25 co-users and 10 kHz carrier frequency offset or Doppler shift by accumulating five or more PN code cycles, using maximum selection detection criterion. Analysis and simulation also shows that cyclic accumulation can improve packet throughput by 50% and by as much as 100% under conditions of high offered traffic and Doppler shift for both fixed capacity and infinite capacity systems

    Sequential detection methods for spread-spectrum code acquisition

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    Novel low cost synchronisation network for spread spectrum systems

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    Spread Spectrum systems are found in many flavours, used in many applications and have existed since the early days of radio communications. The properties of spread spectrum do however place restrictions on the design, and often make the implementation expensive and complex. When using spread spectrum to provide a basic communications infrastructure, many factors need to be considered. These include supplying the appropriate technology at the right cost. To achieve this a trade-off against performance is often required. One of the more difficult aspects of Spread Spectrum design is the synchronisation of the spreading waveform. The primary characteristic of pseudonoise sequence synchronisation is the need for two levels of synchronisation namely acquisition (course synchronisation) and tracking (fine synchronisation). In these networks (the term network is used to describe a circuit or system throughout the thesis.) a decision is required to switch between the two synchronisation modes. The two layer structure of the typical pseudonoise sequence synchronisation network can increase the overall cost of spread spectrum systems. The objective of the research was therefore to find solutions to reduce the overall cost and complexity of the synchronisation network. The synchronisation structure should perform acquisition and tracking in a single structure, and thereby be low cost. To achieve the primary objective of this dissertation a. mixture of theory, simulations and practical implementation was used. The basis of the investigation was a time-variant spectral evaluation of pseudonoise sequences. It is shown that by multiplying a differentiated pseudonoise sequence with another pseudonoise sequence, useful information is obtained that can form the basis of a synchronisation network

    Receiver synchronisation techniques for CDMA mobile radio communications based on the use of a priori information

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    Receiver synchronisation can be a major problem in a mobile radio environment where the communication channel is subject to rapid changes. Communication in spread spectrum systems is impossible unless the received spreading waveform and receiver-generated replica of the spreading waveform are initially synchronised in both phase and frequency. Phase and frequency synchronisation is usually accomplished by performing a two-dimensional search in the time/frequency ambiguity area. Generally, this process must be accomplished at very low SNRs, as quickly as possible, using the minimum amount of hardware. This thesis looks into techniques for improving spread spectrum receiver synchronisation in terms of the mean acquisition time. In particular, the thesis is focused on receiver structures that provide and/or use a priori information in order to minimise the mean acquisition time. The first part of this work is applicable to synchronisation scenarios involving LEO satellites. In this case, the receiver faces large Doppler shifts and must be able to search a large Doppler ambiguity area in order to locate the correct cell. A method to calculate the Doppler shift probability density function within a satellite spot-beam is proposed. It is shown that depending on the satellite’s velocity and position as well as the position of the centre of the spot-beam, not all Doppler shifts are equally probable to occur. Under well defined conditions, the Doppler pdf within the spot-beam can be approximated by a parabola-shaped function. Several searching strategies, suitable for the given prior information, are analysed. The effects on the mean frequency searching time are evaluated. In the second part of the thesis a novel acquisition technique, based on a fast preliminary search of the ambiguity area, is described. Every cell of the ambiguity area is examined two times. The first search is a fast straight line serial search, the duration of which is a crucial parameter of the system that must be optimised. The output of the first search is then used as a priori information which determines the search strategy of the second and final search. The system is compared with well known active acquisition systems and results in a large improvement in the mean acquisition time. Its performance is evaluated in Gaussian and fading Rayleigh channels

    Adaptive CFAR PN Code Acquisition for DSSS Systems

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    The communication between transmitter and receiver in Direct Sequence Spread Spectrum (DSSS) systems starts with synchronisation, which can be carried out in two steps: acquisition and tracking. Acquisition is the coarse searching of the delay of PN code in transmitted signal, and tracking is to find the exact delay of PN code in transmitted signal and maintain the alignment of the two PN codes.This thesis “Adaptive PN code Acquisition for DSSS Systems” presents research on PN code acquisition in DSSS systems. The research focused on the adapitve threshold optimisation with Constant False Alarm Rate (CFAR) techniques in different noise background. Both homogeneous and non-homogeneous noise background are analysed to check the performance of different CFAR techniques, in the terms of Probability of detection ( Pd), Probability of false alarm (Pja) and Mean Acquisition Time (MAT). Thelimitations of general CFAR techniques in non-homogeneous noise background are disclosed in the research, and adaptive censoring technique is applied into general CFAR techniques, showing significant improvement in performance. In the research, MATLAB is used for mathematical simulations, and Monte Carlo simulation is used for independent validation of the theoretical results obtained. ISE, Modelsim, and System generator are used for the hardware implementation in Field Programmable Gate Array (FPGA).Results show that all the kinds of CFAR techniques perform well in homogeneous noise background, with high Pd and short MAT, however, the general CFAR techniques without automatic censoring suffer serious degradation in non-homogeneous noise background. In this thesis, after disclosing the limilation of general CFAR techniques, Greatest-Of/ Smallest-Of CFAR (GO/SO-CFAR) was introduced to solve the problem in non-homogeneous noise background. The simulation results show that GO/SO-CFAR has much better performance than the general CFAR in non-homogeneous noise background, especially in noise background with high interferences, GO/SO-CFAR can maintain high Pd and short MAT. FPGA is used to analyse the complexity of achievement for GO/SO- CFAR detector, and the results illustrate that GO/SO-CFAR is only slightly more complex and slower than the CA-CFAR and OS-CFAR detectors. Therefore, GO/SO- CFAR is much more suitable than general CFAR techniques in non-homogeneous noise background, when the noise condition is unknown

    Coherent peak detection algorithms for utra first stage code acquisition

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    The first stage of UTRA code acquisition is the most crucial stage of the cell search process because it has to deal with the largest amount of uncertainty. We assume that the cell search starts when the mobile station is turned on with no prior information about the base station(s) that may be in the vicinity of the mobile station. The mobile station must acquire slot boundaries in order to obtain timing information of the detected base station. Therefore, this large amount of uncertainty gives longer mean acquisition time. In this thesis, we have devised four new peak detection algorithms for first stage code acquisition. We utilize the available base stations with few multipath signals along with the standard oversampling and pipelining utilization. We compare different coherent and non-coherent combining techniques and run simulations for different carrier frequency errors. We have developed simulation software using MATLAB, to simulate the performance of these algorithms. Our simulation results show that coherent combining for some of the new algorithms provide results close if not better than its non-coherent counterpart even at moderate carrier frequency errors especially at low signal to interference ratio
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